1. Technical Field
This invention relates to audio systems and, more particularly, to voice switching circuits which connect to an audio line for providing two-way voice switched communications.
2. Description of the Prior Art
The use of analog speakerphones have been the primary hands free means of communicating during a telephone conversation for a great number of years. This convenient service has been obtained at the price of some limitations, however. These speakerphone usually require careful and expensive calibration in order to operate in an acceptable manner. They are also designed to operate in a worst-case acoustic environment thereby sacrificing the improved performance that is possible in a better acoustic environment.
The operation of conventional analog speakerphones is well known and is described in an article by A. Busala, "Fundamental Considerations in the Design of a Voice-Switched Speakerphone," Bell System Technical Journal, Vol. 39, No. 2, Mar. 1960, pp 265-294. Analog speakerphones generally use a switched-loss technique through which the energy of the voice signals in both a transmit and a receive direction are sensed and a switching decision made based upon that information. The voice signal having the highest energy level in a first direction will be given a clear talking path and the voice signal in the opposite direction will be attenuated by having loss switched into its talking path. If voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an "at rest" mode which provides the clear talking path to voice signals in a receive direction favoring speech from a distance speaker. In some modern analog speakerphones, if voice signals are not present in either the transmit direction or the receive direction, the speakerphone goes to an idle mode where the loss in each direction is set to a mid-range level to allow the direction wherein voice signals first appear to quickly obtain the clear talking path.
Most high-end analog speakerphones also have a noise-guard circuit to adjust the switching levels according to the level of background noise present. Switching speed is limited by a worst-case time constant that assures that any speech energy in the room has time to dissipate. This limitation is necessary to prevent "self switching", a condition where room echoes are falsely detected as near-end speech. A disadvantage of this type of speakerphone is that no allowance is made for a room that has good acoustics, i.e. low echo energy return and short duration echoes.
With the advent of echo cancelers, echo cancelling speakerphones have become available in the art. These speakerphones are complex and expensive devices that, like the analog speakerphones, attempt to maintain a balance in an inherently unstable environment. The echo cancelling speakerphones available in the art require a user to initiate a white noise start up sequence upon entering each telephone call. This white noise burst into the environment is used by the echo canceller to develop a frequency and phase response for the system loop. From this information, a sampled-time impulse response for the loop is developed, which includes both the room acoustics and the hybrid response. This sampled-time impulse response is a series of signed coefficients that when convolved with the received signal will cancel this signal and yield only the desired transmit signal. In operation, the echo cancellers first determine this impulse response, then invert it by changing the sign of each coefficient. When the received signal is passed through the inverted impulse response, the received signal due to the echoes is cancelled.
A difficulty associated with building suitable echo cancelling speakerphones is that the acoustic environment in which these speakerphones must operate is highly variable and the hybrid environment will change with each call. In an ideal acoustic and hybrid environment, the echo cancellers ought to be able to characterize the system loop based solely on the users speech. Since such an environment cannot generally be assured, the user is required to initiate the disruptive white noise burst at the start of each call to initialize the echo cancellers.
Although each of the above speakerphone systems provide reasonable two-way hands free communications for a user, it is desirable to have an efficient and cost effective speakerphone without the disadvantages and limitations associated with the operation of these systems.